Grid Synchronisation

ABSTRACT

The invention relates to a grid synchroniser for connecting an AC output of a power converter to the AC grid mains. In one aspect the invention provides a grid synchroniser comprising an inverter controller to control an AC output of the inverter, the controller including a receiver to receive grid data from a grid sensor location remote from said inverter. In another aspect we describe techniques for rapid removal of charge from a control terminal of a power switching device such as a MOSFET, IGBT or Thyristor using a particular driver circuit.

FIELD OF THE INVENTION

The invention relates to power generation, and in particular to a gridsynchroniser for connecting an AC output of a power converter to the ACgrid mains. The invention also relates to driver circuits, in particularfor power semiconductor switching devices of the type that are employedin ac (alternating current) inverters. More particularly aspects of theinvention relate to techniques for rapid removal of charge from acontrol terminal of a power switching device such as a MOSFET (MetalOxide Semiconductor Field Effect Transistor) IGBT (Insulated GateBipolar Transistor) or Thyristor.

BACKGROUND TO THE INVENTION

Attempts have been made previously to directly couple switchingsemiconductor devices to the grid in order to maintain phasesynchronisation and simplify system design. One of the key problems ishow to rapidly remove gate charge and at the same time minimise powerloss in the driving circuit. If charge is not removed rapidly from thegate terminal a shoot-through problem results and a ground faultdevelops on the grid. One possible solution is to use small ohmicresistors. However, the use of small ohmic resistors to form potentialdividing circuit often results in huge losses in the resistors andtherefore reduced lifetime and reliability for the system.

Also, many micropower generation systems, such as those in the home,typically comprise one or more of a number of solar cells (e.g. on theroof), wind turbines, combined heat and power systems and other likesystems. The micropower generators generate electricity for the home,and the power is converted into useable voltage and current suitable forthe home, for example 240V at 50 Hz or 110V at 60 Hz. However, morepower than is actually required by the home is sometimes generated. Ifthe micropower generation system were connected to the AC grid, fromwhich power is normally drawn in conventional homes, this surplus powercould be sent back to the AC grid.

However, there exists a need for a system of synchronising the powergenerated by the converters to the power on the grid. Inverters areoften used to generate an AC output from a DC input. The inverters aregenerally located within the proximity of the power source (solar cells,wind turbine etc.). The point at which the inverter is connected to theAC grid mains is often remote from its physical location.

Inverters connected to the grid remotely may experience a phase shiftedline voltage due to line impedance and therefore transfer an increasedamount of reactive power in the network. The increase in reactive powerimplies minimised system efficiency.

There is therefore a need to enable the synchronisation of inverters tothe same line voltage regardless of the line impedance between powerswitchboard and the point of connection of the inverter.

We describe techniques to address the above problems.

SUMMARY OF THE INVENTION Grid Synchronisation

According to the present invention, there is provided a gridsynchroniser to synchronise an AC output of an inverter to an AC gridmains, said inverter having a power input and an AC output forconnection to said AC grid mains to provide a power supply input intosaid AC grid mains, the grid synchroniser comprising: an invertercontroller to control said AC output of said inverter, said controllerincluding a receiver to receive grid data from a grid sensor locationremote from said inverter, said grid sensor sensing a gridcharacteristic of said AC grid mains, wherein said grid data comprisesdata relating to a grid characteristic of said AC grid mans sensed by agrid sensor, and wherein said inverter controller controls said ACoutput responsive to said grid data relating to said gridcharacteristic. Preferably, said sensed grid characteristic comprises aphase of said AC grid mains.

Therefore, the communication of the sensed grid characteristics of theAC grid mains (for example the phase of the current and/or voltage),enables the inverter to be controlled in order that its AC output issynchronised with that of the AC grid mains, irrespective of the lineload between the inverter and the grid connection.

In embodiments, said receiver is in wired communication with a gridsensor. Preferably, said wired communication is through a power supplyline. In alternative embodiments, said receiver is in wirelesscommunication with a grid sensor.

In embodiments, the grid synchroniser further comprises an invertersensor to sense an inverter characteristic of said AC output of saidinverter and to transmit inverter data relating to said sensed invertercharacteristic to a grid sensor located remote from said inverter.Preferably, said sensed inverter characteristic comprises one or more ofa phase of said AC output of said inverter, a power output or a powerefficiency of said inverter.

The present invention also provides a method of synchronising an ACoutput of an inverter to an AC grid mains, said inverter having a powerinput and an AC output for connection to said AC grid mains to provide apower supply input into said AC grid mains, the method comprising:sensing a characteristic of said AC grid mains using a grid sensorlocated remote from said inverter; transmitting grid data relating tosaid sensed grid characteristic to said inverter; and controlling saidAC output of said inverter responsive to said grid data relating to saidsensed grid characteristic. Preferably, said sensed grid characteristiccomprises a phase of said AC grid mains.

Sensing and transmitting the sensed grid characteristics of the AC gridmains (for example the phase of the current and/or voltage), enables theinverter to be controlled in order that its AC output is synchronisedwith that of the AC grid mains, irrespective of the line load betweenthe inverter and the grid connection.

Preferably, the method further comprises sensing a characteristic ofsaid AC output of said inverter; and transmitting inverter data relatingto said sensed inverter characteristic to said grid sensor. Inembodiments, said sensed inverter characteristic comprises one or moreof a phase of said AC output of said inverter, a power output or a powerefficiency of said inverter.

The present invention also provides a system for micropower generation,the system comprising: an inverter having a power input and an AC outputfor connection to an AC grid mains to provide a power supply input intosaid AC grid mains; a sensor, remote from said inverter, to sense a gridcharacteristic of said AC grid mains and to transmit data relating tosaid sensed grid characteristic to said inverter, wherein said inverterincludes a receiver to receive said transmitted data, said inverterbeing configured to control said AC output responsive to said datarelating to said sensed grid characteristic. Preferably, said sensedgrid characteristic comprises a phase of said AC grid mains.

Therefore, the communication of the sensed grid characteristics of theAC grid mains (for example the phase of the current and/or voltage),enables the inverter to be controlled in order that its AC output issynchronised with that of the AC grid mains, irrespective of the lineload between the inverter and the grid connection.

Preferably, said sensor is configured to couple to said AC grid mains ata point where said AC output from said inverter is injected into said ACgrid mains.

In embodiments, the system comprising a plurality of said inverters eachinputting power into said AC grid mains at substantially the same point.

In embodiments, the system further comprises: an inverter sensor tosense an inverter characteristic of said AC output of said inverter andto transmit inverter data relating to said sensed invertercharacteristic to a grid sensor located remote from said inverter.Preferably, said sensed inverter characteristic comprises one or more ofa phase of said AC output of said inverter, a power output or a powerefficiency of said inverter.

Driver Circuits and Techniques

According to another aspect of the present invention there is provided adriver circuit for switching on and off a semiconductor switching deviceconnected to an alternating current (ac) power supply, saidsemiconductor switching device having first and second terminals, and aswitching control terminal to control switching between said first andsecond terminals, the driver circuit being configured to derive fromsaid ac power supply a control signal for application to said switchingcontrol terminal of said semiconductor switching device to control saidswitching, said driver circuit comprising: an input to receive a voltagederived from said ac power supply; a reference line for coupling to oneof said first and second terminals of said semiconductor switchingdevice; a rectifier having an input coupled to said input and an output;and a resistive element coupled between said output of said rectifierand said reference line; and a drive output for driving said switchingcontrol terminal of said semiconductor switching device, said driveoutput being coupled to a circuit node between said resistive elementand said output of said rectifier.

In embodiments the resistive element comprises a resistor although theskilled person will appreciate that an FET may also be used as aresistive element. Preferred embodiments of the circuit include avoltage limiting element such as zener diode coupled between the circuitnode and the reference line. This is particularly important when drivinggrid mains. Preferred embodiments also include a potential dividercoupled to the input, the rectifier being coupled to an output of thepotential divider. In embodiments the resistive element described abovehas a resistance of less than ⅕, more preferably less than 1/10 or lessthan 1/20 of a resistance value of an arm of the potential divider.

In embodiments of the circuit the semiconductor switching devicecomprises a MOSFET, IGBT, or Thyristor, more particularly a power device(that is a device with an operating or switching voltage capability ofgreater than 100 volts and/or a power rating of greater 1 watt).

In some preferred embodiments the ac power supply comprises a grid mainspower supply and the semiconductor switching device has a breakdownvoltage of at least 100 volts. The grid mains power supply may either bea domestic mains power supply such as a 110 volt or 230 volt powersupply or a three phase power supply, typically operating at 415 volts.

The invention further provides a full-bridge or half-bridge rectifiercircuit including one or more semiconductor switching devices andrespective driver circuits as described above.

The invention further provides a power conditioning circuit with a dcinput and an ac output for connection to an ac grid mains power supply.Then embodiments of the above-described driver circuit may be employedto drive a semiconductor switching device chopping a power supplyderived from the dc input to provide an ac output to the grid mainssupply. Some preferred embodiments of such a power conditioning circuithave two semiconductor switching devices driven by respective drivercircuits, switching in alternate half cycles of the ac grid mains powersupply.

Thus one or more driver circuits and switching devices as describedabove may be employed as one or more switches in a dc-to-ac powerconverter of a type described below:

A dc-to-ac power converter, the converter including a transformer havinga primary and a secondary winding, the primary winding of saidtransformer being coupled to a dc input of said power converter and thesecondary winding of said transformer being coupled to an ac output ofsaid converter, and wherein the converter further comprises: a firstpair of switches on said primary side of said converter, coupled betweensaid dc input and said primary winding, to convert a dc supply from saiddc input to an ac current for driving said transformer; a second pair ofswitches on said secondary side of said converter coupled between saidsecondary winding and said ac output, one in a forward path to said acoutput and one in a return path from said ac output; a diode coupledacross each of said secondary side switches; and a controller configuredto control said primary and secondary side switches to convert a dcsupply at said dc input to an ac supply at said ac output.

A DC-to-AC power converter, the converter including a transformer havinga primary and a secondary winding, the primary winding of thetransformer being coupled to a dc input of the power converter and thesecondary winding of the transformer being coupled to an ac output ofthe converter, and wherein: a first and second switch connected to theprimary winding of the transformer to convert a dc supply from the dcinput to an ac current for driving the transformer; a first and secondswitch connected to the secondary winding of the transformer such thatthe first switch is in a forward path to the ac output and the secondswitch is in a return path to the ac output; a first and second diodecoupled across the respective first and second switches connected to thesecondary winding; and wherein the first switch connected to the primarywinding of the transformer is controlled to provide a first half cycleof an ac voltage to the primary winding of the transformer; the secondswitch connected to the primary winding of the transformer is controlledto provide a second half cycle of an ac voltage to the primary windingof the transformer; and the first and second switches connected to thesecondary winding of the transformer as switched to alternately conductthe first and second half cycles of the signal coupled from the primarywinding of the transformer to the secondary winding of the transformer.

Further details of such circuits can be found in the applicant'sco-pending UK and US patent applications GB 0612859.9 filed 29 Jun. 2006and U.S. Ser. No. 11/771,593 filed 29 Jun. 2007 (both of which arehereby incorporated by reference in their entirety).

In a related aspect there is further provided a method of removingcontrol terminal charge from a power semiconductor switching device, themethod comprising supplying a drive signal to said control terminal ofsaid power semiconductor switching device via a rectifier, and leakingcurrent from said control terminal to a reference line whilst said powerswitching device is turned on.

In embodiments of the method the power semiconductor switching devicecomprises a MOSFET, IGBT or Thyristor; in embodiments the reference linecomprises a ground line.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,by way of example only, with reference to the accompanying figures inwhich:

FIG. 1 shows a typical setup of an inverter connected to the AC gridmains;

FIG. 2 shows the phasor diagram of the relationship between Vg and Vg′;

FIG. 3 illustrates a system with multiple inverters;

FIG. 4 shows the system according to the present invention;

FIG. 5 shows the resulting desirable phasor relationship between thegrid voltage and the inverter current for a single inverter network;

FIG. 6 shows a system of multiple inverters connected to the gridaccording to the present invention;

FIG. 7 shows a circuit diagram of a driver circuit according to anembodiment of the invention;

FIG. 8 shows a circuit diagram of a driver circuit omitting a rectifier,illustrating operation of the circuit of FIG. 7;

FIG. 9 shows graphs of effective input voltage to the driver circuitover a half-cycle of ac grid mains for circuits with and without arectifier and lacking a zener diode (upper) and with a zener diode(lower);

FIG. 10 shows, schematically, leakage capacitances of a MOSFET;

FIG. 11 shows measured waveforms from a circuit similar to thatillustrated in FIG. 8;

FIG. 12 shows waveforms from circuits similar to that illustrated inFIG. 7;

FIG. 13 shows a circuit of a dc-to-ac power converter comprising fourswitches (two on a transformer primary side, two on a transformersecondary side) incorporating secondary-side driver circuits accordingto an embodiment of the invention;

FIG. 14 illustrates an example of a full-bridge rectifier circuitincorporating switching devices with switching controlled by an ac gridmains supply to which the circuit is connected, for either receivingpower from a mains supply and delivering power to a load or forreceiving a dc power input and providing an ac power output to a mainssupply; and

FIG. 15 shows waveforms illustrating the operation of FIG. 14.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Grid Synchronisation

FIG. 1 shows a typical setup of an inverter connected to the AC gridmains. The line connecting an inverter to the grid has both inductanceL_L and dc resistance R_L associated with it and whose combined effectform line impedance Z_L. The line impedance is dependent on the lengthof the cable and the conductivity of the material. Due to the lineimpedance, the phase and magnitude of the grid voltage Vg differs fromthe phase and magnitude of the voltage Vg′ that the inverter detects atits terminals. The difference between Vg and Vg′ are dependent on thevalue of Z_L. For inverters connected to the grid and situated remotelysuch as on rooftop the impedance Z_L may be significant. The effect ofthis is that the inverter transfers current in phase with Vg′ and notVg.

FIG. 2 shows the phasor diagram of the relationship between Vg and Vg′,the inverter internal voltage V_i used to control current injection andthe inverter output current Ig′. L_i and R_i represent the inverterinternal impedance though which power is transferred onto the grid.

Vg′=Vg−Ig′(jw L _(—) L+R_L)   1

The angle A represents the difference in phase between Vg and Vg′. Theresult of this phase difference is the transfer of reactive powerbetween inverter and the grid. Reactive power is not converted intouseful power but lost through parasitic resistance on the network. As aconsequence the system efficiency is reduced.

This effect of line impedance may be severe in systems with multipleinverters connected to the grid remotely. Each of the inverter mayexperience different values of line impedance and therefore differentvalues of Vg′. FIG. 3 illustrates a system with multiple inverters.

Vg3′=Vg−(Ig1′+Ig2′+Ig3′) Z _(—) L−Ig3′Z _(—)3   2

Vg2′=Vg(Ig1′+Ig2′+Ig3′) Z _(—) L−(Ig1+Ig2′) Z _(—)4−Ig2′Z _(—)2   3

Vg1′=Vg−(Ig1′+Ig2′+Ig3′) Z _(—) L−(Ig1′+Ig2′) Z _(—)4−Ig1′Z _(—)1   4

The three inverters in FIG. 3 all experience different values of gridvoltage, Vg1′, Vg2′ and Vg3′, due to the impedance in the line. Thecurrents Ig1′, Ig2′ and Ig3′ are generated by inverters 1, 2 and 3respectively. The currents are assumed to be in phase with thecorresponding inverter voltages. Equations 2, 3 and 4 show therelationships between each of the inverter voltages and the gridvoltage.

FIG. 4 shows the proposed solution to the problem of reactive powertransfer. A grid sensor, located at, or substantially near to the pointat which power from the converter is injected onto the AC grid mains, isused to detect a number of characteristics of the AC grid mains, forexample the current and/or grid voltage phases and frequency. Thisinformation is then communicated to the inverter, which is then used tocontrol the current phase of the inverter such that the output currentof the inverter is substantially synchronised to (i.e. in phase with)the voltage Vg.

Therefore, the communication of the sensed characteristics of the ACgrid mains, namely the phase of the current and/or voltage, enables theinverter to be controlled in order that its AC output is synchronisedwith that of the AC grid mains. The object of control is the linecurrent Ig flowing into the grid. The current is detected at theterminals of the grid supply and is therefore corrected from the effectsof impedance on the system.

FIG. 5 shows the resulting desirable phasor relationship between thegrid voltage and the inverter current for a single inverter network.

The communication system can be implemented either with the employmentof a wireless network or a wired network. For example, in a wirednetwork, low rate data may be sent down the power lines throughout thehouse ring main. In a wireless network, a radio protocol such as ZigBee,may be employed to communicate the data between the sensor and theinverter.

Many grid connected inverters come with some sort of communicationcapability already build in for data acquisition and fault diagnostic.It would be possible to build a synchronisation capability over theexisting protocol in order to minimise costs. Alternatively, newprotocols may be developed to communicate such data.

The grid sensor could be remote or local to the inverter. However, thegrid sensor is preferably located at, or substantially near the point atwhich power from the output of the inverter is injected into the AC gridmains, for example at the house's switchboard. The sensor may beintegrated into the switchboard in order to acquire phase and magnitudeinformation of the grid current and voltage.

FIG. 6 shows a system of multiple inverters connected to the grid, andusing the system of the present invention. The AC outputs of each of theinverters in the system can be synchronised to Vg regardless of the lineimpedance. As can be seen, each of the inverters has a receiver (hereshown as a wireless receiver; the skilled reader would understand that awired connection could be used instead) for receiving data from a singlegrid sensor located at, or substantially near to the point at whichpower generated from the inverters is injected into the AC grid mains.Each of the inverters is controlled in response to the data provided bythe single grid sensor.

In an alternative embodiment, additional data may also be captured atthe inverter, such as the output voltage and/or current, the DC or ACpower input, the AC output, the efficiency of the inverter and othersuch data.

In another embodiment, the system could transmit such data from theinverter over the wired or wireless link back to the grid sensor. Suchdata would, for example, enable the grid sensor to detect if one or moreof the inverters was malfunctioning, and to alert a user that action isrequired to correct such a fault.

In embodiments, the grid sensor arrangement could also collate datacollected from the gird and/or the inverters and display such data to auser on a display. Such a display of data would enable a user tovisualise that power being provided by the converter(s), the efficiencyof the converts and/or how much power is being sent back to the grid atany time.

Driver Circuits and Techniques

We also describe a driver circuit that switches on and off asemiconductor device connected to the utility grid by using the gridvoltage as the switching signal. The driver circuit addresses theaforementioned problem associated with removing charge quickly from thegate terminal of the semiconductor device when the grid voltage changespolarity. The rapid removal of charge from the gate terminal enablesgrid connection of devices with high gate charge density that aretherefore slow switching. This in turn leads to the transfer of highpower densities through the switching device. The driver circuit can beused in energy conversion systems such as solar photovoltaic and windand in rectifier circuits connected to the utility grid or similaralternating current supplies.

FIG. 7 shows the circuit solution. The circuit uses high value ohmicresistor R1 and therefore affords low potential divider losses. A lowvalue resistor R3 is used to enable rapid removal of gate charge as thegrid voltage drops to zero. The semiconductor switch T can beimplemented as a MOSFET device, IGBT or Thyristor.

Principle of Operation

Assume the grid voltage Vgrid is zero, implying that the potentialdifference between point 1 and 2 in FIG. 1 is zero, and the potentialdifference between 2 and 5 is zero. Also assume the zener diode Z has avalue Vz. The resistors R1 and R2 are chosen to have high value, example1 mega ohm each. When the grid voltage rises from zero, that is apositive potential difference develops between 1 and 2, the voltage atpoint 4, V4, also increases. The diode D becomes forward biased andbegins to conduct as V4 rises. The gate voltage Vgate, that is thepotential between 3 and 5, therefore increases due to diode charge. Asthe grid voltage continues to rise, so does Vgate. The transistor Tturns on when Vgate equals the turn on threshold of T. Vgate staysconstant at Vz even when Vgrid rises further.

Vgate stays constant until V4 drops below Vz as Vgrid drops. There aretwo possibilities to the state of D when V4 drops below Vz. If the gatecharge is removed rapidly, D would remain forward biased until Vgridbecomes zero. In this case T turns off before the Vgrid reversespolarity. On the other hand if the Vgate remains higher than V4, D isreverse biased. In this case there is a possibility that the MOSFET ison when Vgrid reverses. If this happens in, say, a half or full-bridgerectifier, the result is a short circuit in the power circuit.

To remove charge rapidly, the value of resistor R3 is chosen to have alow value, for example 20 kilo ohm to 100 kilo ohm. This would enablethe removal of charge rapidly and therefore enable high gate chargeswitches to be used. It is possible to have a low value of R3 acrossVgate because of the blocking diode D. If D is short-circuited (as shownin FIG. 2, which is included to illustrate this) Vgate may not attain Vzfor all or part of the half grid cycle and therefore the switch wouldnot work properly. An example for the value of Vz that would result innormal operation of the switch is say 15V (this varies with the type ofswitching device). In mains driven circuits the zener diode isimportant. FIG. 8 illustrates a circuit with D replaced by a conductingwire.

In this case the resistors R2 and R3 form a parallel network with valueequal to R2*R3/(R2+R3). FIG. 3 shows the resulting gate drive signalswith and without the diode D connected.

FIG. 9 shows graphs of effective input voltage to the driver circuitover a half-cycle of ac grid mains for circuits with and without arectifier and lacking a zener diode (upper) and with a zener diode(lower).

Gate Charge and Shoot-Through

The gate drive circuit has the ability to discharge the gate terminal ofthe connected switching device rapidly, therefore preventingshort-circuiting the grid when the grid voltage reverses. Switchingdevices such as MOSFET have parasitic gate capacitances that storecharge. FIG. 10 shows a representation of the MOSFET with drain-to-gateand gate-to-source parasitic capacitors.

The charge stored in the combined capacitance C1 and C2 is dischargedthrough R3 and through some leakage current in the MOSFET and the zenerdiode. The time constant for the discharge assuming that the diode Dstays reverse biased is given by equation 1.

T=1/(C R3+C RL)

were C is the overall gate capacitance and RL is the leakage resistancedue to the MOSFET and zener diode. This equation also indicates that asmall R3 reduces T.

FIG. 11 shows some experimental results obtained when the gate circuitis designed without the diode D and the resistor R3. In the figure, itcan be observed that the falling gate voltage overshoots thezero-crossing point of the grid by a significant amount to cause shortcircuit when the other half cycle rises (see the double-ended arrow).FIG. 12 shows the results obtained when D and R3 are included in thedesign. In this case the gate signals fall rapidly enough to avoid anysignificant overshoot.

APPLICATION EXAMPLES

The driver circuits can be used in applications where synchronisedswitching of the grid is used for power transfer in either direction.One example is as used in the circuit diagram of FIG. 13. The principleof operation of this circuit is described in our earlier patentapplication (ibid). In this circuit the drivers switches the two IGBTsin alternate half cycles to allow power transfer from a source such assolar photovoltaic energy.

FIG. 14 shows another application of the proposed driver circuit. Inthis case power can be transferred from the grid to the load or the loadcan supply power to the grid. FIG. 15 illustrates the waveformsappearing across the load. The amplitude difference between Vg and VLare for illustration clarity.

No doubt many other effective alternatives will occur to the skilledperson. It will be understood that the invention is not limited to thedescribed embodiments and encompasses modifications apparent to thoseskilled in the art lying within the spirit and scope of the claimsappended hereto.

1. A grid synchroniser to synchronise an AC output of an inverter to anAC grid mains, said inverter having a power input and an AC output forconnection to said AC grid mains to provide a power supply input intosaid AC grid mains, the grid synchroniser comprising: an invertercontroller to control said AC output of said inverter, said controllerincluding a receiver to receive grid data from a grid sensor locationremote from said inverter, said grid sensor sensing a gridcharacteristic of said AC grid mains, wherein said grid data comprisesdata relating to a grid characteristic of said AC grid mans sensed by agrid sensor, and wherein said inverter controller controls said ACoutput responsive to said grid data relating to said gridcharacteristic.
 2. The grid synchroniser according to claim 1, whereinsaid sensed grid characteristic comprises a phase of said AC grid mains.3. The grid synchroniser according to claim 1, wherein said receiver isin wired communication with a grid sensor.
 4. The grid synchroniseraccording to claim 3, wherein said wired communication is through apower supply line.
 5. The grid synchroniser according to claim 1,wherein said receiver is in wireless communication with a grid sensor.6. The grid synchroniser according to claim 1, further comprising: aninverter sensor to sense an inverter characteristic of said AC output ofsaid inverter and to transmit inverter data relating to said sensedinverter characteristic to a grid sensor located remote from saidinverter.
 7. The grid synchroniser according to claim 6, wherein saidsensed inverter characteristic comprises one or more of a phase of saidAC output of said inverter, a power output or a power efficiency of saidinverter. 8-11. (canceled)
 12. A system for micropower generation, thesystem comprising: an inverter having a power input and an AC output forconnection to an AC grid mains to provide a power supply input into saidAC grid mains; a sensor, remote from said inverter, to sense a gridcharacteristic of said AC grid mains and to transmit data relating tosaid sensed grid characteristic to said inverter, wherein said inverterincludes a receiver to receive said transmitted data, said inverterbeing configured to control said AC output responsive to said datarelating to said sensed grid characteristic.
 13. The system according toclaim 12, wherein said sensed grid characteristic comprises a phase ofsaid AC grid mains.
 14. The system according to claim 12, wherein saidsensor is configured to couple to said AC grid mains at a point wheresaid AC output from said inverter is injected into said AC grid mains.15. The system according to claim 14, comprising a plurality of saidinverters each inputting power into said AC grid mains at substantiallythe same point.
 16. The system according to claim 12, furthercomprising: an inverter sensor to sense an inverter characteristic ofsaid AC output of said inverter and to transmit inverter data relatingto said sensed inverter characteristic to a grid sensor located remotefrom said inverter.
 17. The system according to claim 16, wherein saidsensed inverter characteristic comprises one or more of a phase of saidAC output of said inverter, a power output or a power efficiency of saidinverter.
 18. A driver circuit for switching on and off a semiconductorswitching device connected to an alternating current (ac) power supply,said semiconductor switching device having first and second terminals,and a switching control terminal to control switching between said firstand second terminals, the driver circuit being configured to derive fromsaid ac power supply a control signal for application to said switchingcontrol terminal of said semiconductor switching device to control saidswitching, said driver circuit comprising: an input to receive a voltagederived from said ac power supply; a reference line for coupling to oneof said first and second terminals of said semiconductor switchingdevice; a rectifier having an input coupled to said input and an output;and a resistive element coupled between said output of said rectifierand said reference line; and a drive output for driving said switchingcontrol terminal of said semiconductor switching device, said driveoutput being coupled to a circuit node between said resistive elementand said output of said rectifier.
 19. The driver circuit as claimed inclaim 18 further comprising a voltage limiting element coupled betweensaid circuit node and said reference line.
 20. The driver circuit asclaimed in claim 18 further comprising a potential divider coupled tosaid input, and wherein said rectifier is coupled to an output of saidpotential divider.
 21. The driver circuit as claimed in claim 20 whereinsaid resistive element has a resistance value of less than one fifth aresistance value of an arm of said potential divider.
 22. The drivercircuit as claimed in claim 20 wherein said resistive element has aresistance value of less than one tenth a resistance value of an arm ofsaid potential divider.\
 23. The driver circuit as claimed in claim 18wherein said semiconductor switching device comprises a MOSFET, IGBT orThyristor.
 24. The driver circuit as claimed in claim 18 wherein said acpower supply comprises a grid mains power supply, and wherein saidsemiconductor switching device comprises a power semiconductor switchingdevice with a breakdown voltage of at least 100 volts. 25-31. (canceled)